The present invention relates to silicone rubber-based impact modifiers in the form of polymerized alkenyl monomer-containing grafts, such as styrene and acrylonitrile, of emulsion polymerized silicone rubber particles having an average particle size in the range of about 400 nm or more. More particularly, the present invention relates to the employment of such polymerized alkenyl containing impact modifiers in thermoplastic resins to provide thermoplastic compositions having enhanced impact strength and improved weatherability.
As shown by Craig, U.S. Pat. No. 5,726,270, which is incorporated herein by reference, aqueous dispersions of organopolysiloxanes are provided in the form of monomodal organopolysiloxane particles having a pre-determined particle size of up to about 2 microns. As discussed in U.S. Pat. No. 5,726,270, current. manufacturing practices for making organopolysiloxane dispersions often emulsify pre-existing organopolysiloxane fluids or gums under high shear conditions. Alternative procedures include batch stirring siloxane precursors in water in the presence of a surfactant. Additional emulsion polymerization methods are taught, such as shown in U.S. Pat. No. 2,891,920, which describes the use of a base catalyst with a cationic surfactant, and J.P.62141029 A2 870624, which is directed to a continuous addition of a pre-emulsion cyclo-siloxane precursor.
While various procedures are available for making aqueous polysiloxane emulsions having average particle sizes of about 10 to 300 nm, these dispersions often have been restricted to such applications as personal care, adhesives and coatings where small particles sizes often provide advantages.
It would be desirable therefor to be able to make aqueous dispersions of emulsion polymerized silicone rubber particles to expand the utility of aqueous polysiloxane emulsions to silicone emulsion rubbers having an average particle size of about 400 nm, or greater.
It also would be desirable to provide low temperature impact modifiers in the form of polymerized alkenyl monomer-containing grafts of such emulsion polymerized silicone rubber particles.
In addition, it would be desirable to provide thermoplastic compositions comprising a thermoplastic polymer and an effective amount of an impact modifier in the form of a polymerized alkenyl monomer-containing graft of an emulsion polymerized silicone rubber having an average particle size of 400 nm, or greater.
The present invention is based on the discovery that silicone rubber particles having a volume average particle size of 400 nm or greater can be made by a single stage semi-continuous process involving the emulsion polymerization of siloxane precursors under low shear, substantially non-homogenizing conditions.
As used hereinafter, the expression xe2x80x9csemi-continuous processxe2x80x9d means the introduction under emulsion polymerization conditions of silicone rubber siloxane precursors, such as octamethylcyclotetrasiloxane and xcex3-mercaptopropyltrimethoxysilane into a reactor over an extended period of time, for example, about 2 to about 12 hours, and preferably, about 4 to about 8 hours.
The expression semi-continuous process also includes the employment of mild, and/or low shear non-homogenizing conditions during the emulsion polymerization of the silicone rubber siloxane precursors. The degree of agitation used during the semi-continuous processing of the silicone rubber siloxane precursors substantially minimizes the formation of silicone rubber particles having an average particle size of below about 400 nm.
The resulting silicone rubber particles can thereafter be respectively grafted with a polymerizable alkenyl monomer, such as a vinyl monomer, to form a polymerized alkenyl polymer shell, to provide valuable impact modifiers for a variety of thermoplastic polymers, such as polyesters, polycarbonates, polyestercarbonates, polyimides, polyetherimides, and polyamides.
There is provided by the present invention, a method for making an aqueous silicone rubber latex, which comprises,
(1) semi-continuously adding silicone rubber siloxane precursors into a reactor to provide contact under agitation with a reaction mixture comprising water and an acid catalyst-surfactant at a temperature in the range of about 30xc2x0 C. to about 110xc2x0 C., and
(2) recovering a silicone rubber latex comprising silicone rubber particles having a volume average particle size in the range of about 400 nm to about 2 microns.
There is also provided, a method for making a silicone rubber-based graft copolymer comprising,
(1) effecting reaction at a temperature of about 50xc2x0 C. to about 95xc2x0 C. between (A) an aqueous silicone rubber latex having a pH of about 3 to about 9, and comprising silicone rubber particles having a volume average particle size in the range of about 400 nm to about 2 microns, and (B) an aqueous mixture comprising at least one polymerizable alkenyl organic monomer, where a sufficient proportion of mixture (B) is utilized in the reaction to provide from about 15% to about 75% by weight of alkenyl polymer shell, based on the total weight of graft copolymer, and
(2) coagulating the resulting latex from (1), and
(3) recovering, washing, and thereafter drying the resulting solids.
There is further provided, a method for making a silicone rubber-based graft copolymer comprising,
(1) effecting reaction at a temperature of about 50xc2x0 C. to about 95xc2x0 C. between (A) an aqueous silicone rubber latex having a pH of about 3 to about 9, and comprising silicone rubber particles having a volume average particle size in the range of about 400 nm to about 2 microns, and (B) an aqueous mixture comprising, styrene and acrylonitrile in a weight ratio of between about 90:10 to about 50:50, where a sufficient proportion of mixture (B) is utilized in the reaction, to provide from about 15% to about 75% by weight of alkenyl polymer shell, based on the total weight of graft copolymer, and
(2) coagulating the resulting latex from (1), and
(3) recovering, washing, and thereafter drying the resulting solids.
There is still further provided by the present invention, a thermoplastic blend comprising (C), thermoplastic polymer, and (D) about 5% to about 50% by weight, based on the weight of thermoplastic blend, of a silicone rubber graft copolymer having about 15% to about 75% by weight of alkenyl polymer shell, and silicone rubber particles having a volume average particle size in the range of about 400 nm to about 2 microns.
Still another aspect of the present invention is directed to a method for preparing a thermoplastic blend, which comprises mixing (C), thermoplastic polymer, and (D) about 5% to about 50% by weight, based on the weight of thermoplastic blend, of a silicone rubber graft copolymer having about 15% to about 75% by weight of alkenyl polymer shell, and silicone rubber particles having a volume average particle size in the range of about 400 nm to about 2 microns.
In the practice of one form of the method of the invention, an emulsion polymerized silicone rubber latex is initially formed by semi-continuously adding to a reactor containing water, which is being agitated, such as by stirring, at a temperature in the range of about 30xc2x0 C. to about 110xc2x0 C., and preferably about 75xc2x0 C. to about 95xc2x0 C., a mixture of silicone rubber monomers. The semi-continuous addition of monomers can be effected, stepwise, and in a dropwise manner, over a period of up to about 24 hours. An effective amount of a surfactant can be used initially in the reactor as part of the agitated aqueous mixture, or it can be introduced with the silicone rubber monomers.
Among the surfactants which can be used, dodecylbenzenesulfonic acid is preferred. Surfactants which can be used in the practice of the invention include acid catalyst-surfactants, for example, sulfonic acids, such as alkyl-, and alkaryl-arylsulfonic acids and mixtures of surface-active sulfonic acid salts with strong mineral acids. Additional sulfonic acid catalysts/surfactants are shown in U.S. Pat. No. 3,294,725, and Craig, U.S. Pat. No. 5,726,270 which are incorporated herein by reference.
Various silicone rubber monomers can be used to form the initial emulsion polymerized silicone rubber latex used in the practice of the invention. Some of the preferred silicone rubber monomers include cyclosiloxanes, such as octamethylcyclotetrasiloxane, as shown for example in the Encyclopedia of Polymer Science and Engineering, Volume 15, 2nd Edition, pp. 205-308, (1989), John Wiley and Sons. Cross-linking silanes include trifunctional such as trimethoxymethylsilane, and triethoxyphenylsilane, and tetrafunctional, for example, tetraethoxysilane. The cross-linking silanes can be used at from about 0.1% to 30% by weight of the silicone rubber monomer mixture. Useful emulsion polymerizable silicone rubber monomers are for example cycloalkylsiloxanes, such as hexamethylcyclotrisiloxane, or octamethylcyclotetrasiloxane which can be copolymerized with from about 0.1% to about 30% by weight of a cross-linking agent. Suitable cross-linking agents are for example, tetraalkoxysilane, such as, tetraethoxysilane, and in further combination with an alkylacryloxyalkyldialkoxyalkylsilane, as illustrated by xcex3-methacryloxypropyldimethoxymethylsilane. A comprehensive list of silicone rubber monomers can be found in xe2x80x9cSiliconesxe2x80x9d, Hardman and Torkelson, Encyclopedia of Polymer Science and Engineering, volume 15, 2nd Edition, pp. 205-308, (1989), John Wiley and Sons, which is incorporated herein by reference.
In preparing the graft of the emulsion polymerized silicone rubber latex, a suitable polymerizable alkenyl monomer, alone, or in combination as a mixture of two or more alkenyl monomers, such as styrene, triallyl cyanurate, acrylonitrile, and methylmethacrylate, can be used in combination with the rubber latex. When a mixture of styrene and acrylonitrile is used, then their weight ratio is between about 90:10 to about 50:50.
The proportion of alkenyl monomer and emulsion polymerized rubber latex can vary widely by weight. For example, there can be used by weight, from about 15% to about 75% alkenyl monomer, based on the total weight of graft copolymer.